1. Field of the Invention
This invention relates to an image compression system wherein image signals are encoded adaptively into variable-length codes according to combinations of sum components of two adjacent picture elements with difference components thereof.
2. Description of the Prior Art
It is well known that the information transmission capacity requirement can be reduced or band compression can be achieved by eliminating signals which are redundant from the viewpoint of statistical properties of images or which are unnecessary from the viewpoint of visual characteristics.
When biases are imparted to an image by conducting an image transformation by utilizing strong spatial correlation between adjacent picture elements of the image, it becomes possible to conduct encoding of image signals at a high efficiency. As to transformation systems for imparting biases to an image, there have heretofore been known a linear prediction transformation system and an orthogonal transformation system. The linear prediction transformation system utilizes statistical biases exhibited by prediction error components when adjacent picture elements are predicted. The orthogonal transformation system utilizes a bias arising in a spectrum of each frequency component which is outputted when an image is transformed into a spatial frequency zone. In general, in order to achieve high-efficiency encoding, the amcunt of information is reduced by imparting a distortion, i.e. a quantization distortion, within a visually permissible range to the transformation components. Further, in the case where biases exist in the probability distributions of the transformation components, the entropy becomes small, and it is advantageous to input variable-length codes suitable for the small entropy.
On the other hand, the aforesaid statistical biases are markedly different depending on the type of the image. Thus the biases are large in the case of a smooth image and, conversely, large fluctuation arises in the case of a complicated image. As a result, in the variable-length encoding, the compressibility is markedly different according to the type of the image. That is, the amount of image information compressed is markedly different depending on the type of the image. This means that, when the speeds of input and output of image signals are constant in a system, variable-length encoding conducted in this system causes a difference in the time base to arise with respect to the amount of information transmitted. Accordingly, it becomes necessary to correct the fluctuation of the time base.